Energy supply, demand and delivery infrastructure systems present current and growing societal issues with emphasis on low cost, efficient and environmentally sensitive solutions. The energy industry, regulatory groups, and government agencies seek to provide safe, efficient and affordable energy to consumers. In the wake of digital and electronic device ubiquity, population growth, industry, and personal comfort concerns, energy demand is rising sharply.
For example, in Texas, summer daytime energy demands can frequently exceed the International Organization for Standardization's (ISO) rated capacity for power-generators. This is due to the high temperatures and resulting increased demands from air-conditioners and other devices, as well as a decrease in energy production efficiency. An independent system operator (the Electric Reliability Council of Texas (ERCOT)) manages the flow of electricity in Texas and is one of nine such system operators within the United States. ERCOT has implemented measures to increase the variable system-wide offer cap (SWOC) to $7,000/MWh in June, 2014, and to $9,000/MWh in June, 2015, in an attempt to attract development of new generation. To date, however, this has not succeeded. Regulators and the energy industry have undertaken a variety of other measures to reduce peak daytime energy demands, such as campaigns to encourage less electricity use and optimization of existing generation and delivery infrastructure systems.
One potential efficiency-enhancing measure is inlet-cooling of power-generating devices, such as fuel-fired power-generating turbines. Inlet-cooling refers to the cooling of, for example, air fed to a turbine to lower the temperature of the overall inlet air (“turbine inlet-cooling” or “TIC”). Power-generation efficiency also depends on the mass flow rate of the air to the turbine. An increase in temperature decreases the mass flow rate, because gas density decreases when the temperature increases, as demonstrated by the Ideal Gas Law. An increase in ambient air temperature, such as in the summer, during peak production hours, decreases the power-generation of combustion turbines. Inlet-cooling increases both the air density and the mass flow rate of air to the turbine and, thus, increases the power output. The power output of all combustion turbines decreases as the inlet air temperature increases.
The ISO-rated capacities of combustion turbines are based on standard ambient air conditions of 59° F. and 14.7 psia at sea level. An increase in inlet air temperature from 59° F. to 100° F., such as on a hot summer day, decreases power output of a combustion turbine to about 73% of its ISO-rated capacity. This can lead to a loss of opportunity for power producers to sell more power, just when the rise in ambient temperature increases power demand. Cooling the inlet air from 100° F. to 59° F. prevents loss of 27% of the ISO-rated generation capacity. Cooling the inlet air further, to about 42° F., enhances the power-generation capacity to 110% of the ISO-rated capacity. Therefore, cooling the inlet air from 100° F. to 42° F. can increase the output capacity by about 50%.
However, the need to refrigerate a cooling medium, such as ambient air, to the temperatures desired for inlet-cooling can also reduce the overall increase in power achievable by inlet-cooling. Such refrigeration is typically performed in the same hot, ambient conditions as exemplified above. Conventional systems for inlet-cooling employ water or air-coolers, such as cooling towers, evaporative coolers, and/or absorption chillers, which require relatively high energy inputs in order to refrigerate the inlet-cooling medium.
Evaporative cooling uses the heat of ambient air to evaporate water, taking with it a high latent heat of vaporization, thus cooling the air. The inlet temperatures that evaporative cooling can achieve are significantly limited by the difference between the dry bulb temperature and the wet bulb temperature. Evaporative cooling also requires large amounts of water. In 2005, it was estimated that about 41% of all fresh water in the United States is used for cooling power-generating facilities. The current need for increasing power-generation while conserving natural resources cannot support this ongoing practice. The issue is a focus of the U.S. Environmental Protection Agency (EPA), which recently established new guidelines on the use of fresh water within the power-generation sector.
Absorption cooling, another system for inlet-cooling, operates similarly to conventional compression coolers (air conditioners) in that a refrigerant with a low boiling point is evaporated, using the heat removed from the medium to be cooled. Absorption cooling provides a liquid into which the gaseous refrigerant is absorbed. A heater is subsequently used to separate the refrigerant from the liquid medium. Absorption cooling is limited by the need for environmentally-friendly coolants with sufficient heat transfer and vaporization properties. Additionally, absorption systems are complex and expensive. The power required to operate such systems is estimated to be about 0.28/kW/RT (refrigerated ton).
Alternatively, thermal energy storage (“TES”) is a system which creates chilled water and/or ice pools using low priced electricity during off-peak hours. The coolants can then be used for TIC purposes during times of peak energy demand. Disadvantages of TES are the need for off-peak power to create the ice or chill the water, in addition to large storage volumes to retain the water/ice media, and to sustain the temperature for use during peak times.
Thus, the best known TIC systems include relatively high capital cost, energy input requirements, reliance on fresh water, and inability to effectively operate during peak times without also requiring resources during times of off-peak energy demand.